This invention relates generally to the art of industrial automatic control devices and more specifically to valve position controllers.
It is well known that factories and other industrial installations are increasingly automated. There are, for example, apparatus for automatically and continuously repositioning industrial valves to achieve desired fluid flows. Often desired flows change with changing conditions. For example, it may be desirable to control fluid flow so as to maintain a selected pressure, temperature, flow rate, or level. In fact, many industrial installations include fluid-condition transducers for monitoring flow (as measured by temperature, pressure, flow rate, etc.), with electrical signals issued by such transducers being graduated between 4-20 milliamps (ma). Similarly, many industrial valves include valve position feedback potentiometers, or transducers, sometimes housed in limit switches, for providing signals which are indicative of valve positions (i.e. open, closed, 3% open, etc.). Normally, a valve position signal is used to provide a visual readout as to a valve's position. In some cases a fluid-condition signal is fed to a set point process controller which determines therefrom a new desired-position of an industrial valve element. The set point process controller then sends a "desired-position signal" to a value position controller which adjusts the industrial valve element to the new position by means of a pressurized-fluid system (such as a pneumatic system). Changing the position of the industrial valve element causes a desired change in the condition of the fluid.
It is known in the prior art to provide a valve position controller which employs mechanical/pneumatic feedback from an industrial valve to the pressurized-fluid system so that the pressurized-fluid system stops moving the industrial valve element once the industrial valve element achieves the new desired position ordered by the set point process controller. Such a system is sold by Automax, Inc. under the mark AUTOMAX H-4000. A problem with such a mechanical/pneumatic feedback is that it requires the use of "instrument air" (highly clean air) for both actuating an industrial valve motivator and in a feedback path. In this regard, air used to activate the pneumatic motivator is the same air as must be used in the mechanical/pneumatic feedback. Yet another difficulty with this system is that pneumatic feedback controls are rather imprecise in operation and, if they are made to be highly precise, are expensive to manufacture. Still further, mechanical/pneumatic feedback systems must usually be mounted at the industrial valves and they are therefore difficult to inspect and adjust. Also, they require a continuous flow of air even when no valve adjustment is necessary.
Thus, it is an object of this invention to provide a valve position controller which does not have to be energized by instrument air. Similarly, it is an object of this invention to provide a pressure-activated valve position controller which can be activated pneumatically or hydraulic. Along the same line, it is an object of this invention to provide a valve position controller which is highly precise in its operation but yet which does not require the use of many mechanical parts. It is an object of this invention to provide a valve position controller which does not require a continuous flow of air, or other undue power consumption, when no valve adjustments are necessary.
It is also an object of this invention to provide a valve position controller which can be easily adjusted and which can be easily switched between a manual mode of operation and an automatic mode of operation.
It is suggested in U.S. Pat. No. 5,198,973 to Steutermann that a position transmitter generate a digital binary coded position signal of an industrial valve element. A comparator device compares the binary coded position signal and a desired binary coded command signal and, when there is a mismatch, activates actuating valves to control flow of pressurized fluid to a motivator device for moving the industrial valve element to a desired position. A digital feedback tells the position transmitter when the industrial valve element has achieved the new position. Although Steutermann's apparatus has the advantage of speed, it has a disadvantage in that the position transmitter is limited to producing 100 position signals. That is, a resolution no greater than 100 appears to be possible. It is thought that this limitation is caused by a physical need to mount and separate hall effect sensors in the position transmitter, one for each different position. Yet another difficulty with the digital device is that it is complex, and therefore cannot be easily worked on by average technicians in the field.
Thus, it is an object of this invention to provide a valve position controller which is quite fast in operation yet which provides a high resolution. It is also an object of this invention to provide a valve position controller which is relatively uncomplicated in structure and which can, therefore, be worked on in the field by technicians.
An analog electrical comparator system has been sold by the Virginia Valve Company Corporation under the mark M-92 POSITIONER/CONTROLLER which uses neither a pneumatic feedback signal nor a digital signal for determining new positions of industrial valves, but rather which uses an analog electrical position signal, from a potentiometer for example. In this suggested system a set point process controller receives a signal from a fluid-condition transducer and determines therefrom a desired position of an industrial valve controlling flow through a conduit. The determined desired-position signal is fed to a comparator of a valve position controller which also receives an electrical actual-position signal from a valve position feedback potentiometer coupled to an industrial valve motivator. When there is a difference between the actual-position signal and the desired-position signal the comparator produces control signals which activate electrical opening and closing solenoids for controlling pressurized fluid flow through closing and opening lines to the valve motivator to open and close the industrial valve as required. Once the actual-position signal and the desired-position signal are equal the comparator deactivates the opening and closing solenoids. When the opening and closing solenoids are deactivated, opening and closing actuating valves are biased to move to positions which allow pressurized fluid to flow to both sides of the industrial-valve actuator, thereby locking the industrial-valve motivator in a fail-in-place condition.
Although this system has the advantage of simplicity, it also has some problems. One problem with this system is that when both sides of the motivator are pressurized there is usually some "drift". That is, the motivator does not maintain its position, but rather drifts to a new position.
Another problem with this analog electrical comparator system is that it can be difficult to adjust it to accommodate different resolutions. That is, if the system is adjusted to a high resolution, so as to have 160 possible positions of the industrial valve, for example (with small intervals between adjacent positions), it is difficult to keep the motivator from repeatedly overshooting desired positions and thereby causing oscillations of the pressurized-fluid motivator.
Still another difficulty with such an electrical analog comparator valve position controller is that it cannot be easily switched to a manual mode in which the fluid-pressure motivator is directly controlled (not including the comparator) manually. When the system is placed in such a manual mode the comparator feedback must be disabled. Thus, once the industrial valve element has been manually moved to a new position there is nothing to maintain it there. There will be drift.
It is therefore an object of this invention to provide an electrical, analog, valve position controller which employs a comparator which can be adjusted to operate at resolutions up to 160. It is also an object of this invention to provide such a valve position controller which does not allow an industrial-valve pressure-fluid motivator to drift once the industrial-valve element is in a new position. Yet another object of this invention is to provide such an electrical analog comparator valve position controller which can be easily switched between an automatic mode of operation and a manual mode of operation.
Yet another object of this invention is to provide an electrical analog, valve position controller which, when it is in a manual mode of operation, has positive control over movement of the industrial-valve motivator, but which does not allow any drift when the industrial-valve fluid actuated motivator is turned off.